Neither pipe carries a greater amount of water!!!

At the maximum allowable headwater, both pipes carry about 10 cubic feet per second (4,000 gallons per minute). The only difference is the outlet velocity. In the corrugated pipe the outlet velocity is 6 feet per second (ft /sec) while in the smooth pipe the outlet velocity is over 9 ft /sec!

Grass will provide all the outlet protection needed in the corrugated pipe, however, some rip-rap is needed to provide erosion protection for the smooth pipe.

Why?

Because both pipes are flowing under INLET control. In other words, the critical factor in the capacity of the pipe is how the water gets in the inlet. Although the second pipe is smooth on the inside, the only effect is to increase the velocity.

Which is the better installation?

It all depends on the cost. The smooth pipe may need rip-rap, but may be less likely to plug with debris. The corrugated pipe does not need extra erosion control but may be harder to install due to its increased flexibility.

If you need assistance, contact the Soil and Water Conservation District or the County Highway Department in your county. Ask, don't assume!